Continuous Samples without allowing regeneration (differences LabVIEW/LabWindows CVI)

[jonasz]

Hi,

I am trying to implement continuous analogue waveform generation in a LabWindows/CVI programme.

My application should not be allowed to regenerate samples. While my implementation in LabVIEW similar to the example in Voltage (non-regeneration) - Continuous Output.vi works without problems, I run into issues in LabWindows, namely I either don't generate all the samples I need or I get a buffer underrun:

DAQmxCreateTask("stimAna", &stimOutATH);
DAQmxCreateAOVoltageChan (stimOutATH, "SpikeDAQCard/ao0", "", ANAO_MIN_VOLT, ANAO_MAX_VOLT, DAQmx_Val_Volts, NULL));
DAQmxCfgOutputBuffer (stimOutATH, (uint32_t) myWave.SamplingRate/5);
DAQmxCfgSampClkTiming (stimOutATH, NULL, myWave.SamplingRate, DAQmx_Val_Rising , DAQmx_Val_ContSamps ,(uInt64) myWave.SamplingRate/10));
DAQmxSetWriteAttribute (stimOutATH, DAQmx_Write_RegenMode, DAQmx_Val_DoNotAllowRegen);
DAQmxSetWriteAttribute (stimOutATH, DAQmx_Write_RelativeTo, DAQmx_Val_CurrWritePos);

while (data_left_to_write)
{
	DAQmxWriteAnalogF64 (stimOutATH, chunk_size, false, -1, DAQmx_Val_GroupByChannel, y, &sWritten, NULL);
	if (first_run)
	{
		DAQmxErrChk (DAQmxStartTask(stimOutATH);
	}
}
	
// set data and output to safe values
y[0] = 0; 
DAQmxWriteAnalogF64 (stimOutATH, chunk_size, false, 10, DAQmx_Val_GroupByChannel, y, &sWritten, NULL);
	
// *1
DAQmxWaitUntilTaskDone(stimOutATH, -1);
// *2	
DAQmxStopTask (stimOutATH);

 If I don't wait for the task to be done (*1), I don't generate all the samples of my waveform, specifically, if the waveform to generate is short, only few samples will be generated before reaching DAQmxStopTask (*2). On the other hand, if I do wait until the task is done using (*1), then stopping the task (or indeed any other operation on it) will trigger error -200290 (buffer underrun). 

How should I best handle this situation? And why does the LabVIEW example write all samples before stopping the task?

[Maxwell_Taylor]

Hey jonasz,

I'm including a document that I believe will answer your question.  Take a look at this articleand there is also an example linked to it that you may want to take a look at:

http://digital.ni.com/public.nsf/allkb/A806A0349FBF30CD8625795A007376F3?OpenDocument

Also, LabVIEW  writes all samples before stopping the task because you're using a DAQmxWaitUntilTaskDone function. It will empty the buffer before moving to the Stop Task function.  Take a look at the link below for further explanation.

http://zone.ni.com/reference/en-XX/help/370471W-01/daqmxcfunc/daqmxwaituntiltaskdone/

[jonasz]

Hi Taylor,

Thanks for your answer. I have seen the explanation for how to deal with error -200290. However, this does not address my problem.

My problem is that I cannot use the DAQmxWaitUntilTaskDone function, because using it will throw error -200290 next time I refer to the task (e.g. to stop it).

Another approach to take was to try to query how many items are still left in the write buffer. I used this function:

DAQmxGetWriteAttribute  (task, DAQmx_Write_SpaceAvail, &space);

However, space does not seem to increase after the task was started and remains at 0 no matter how much time passes.

Is there another way to tell how many items are still in the buffer (I know the difference between on-board and PC buffer; PC buffer would be enough)?

Thanks,

Jonas

[Maxwell_Taylor]

Hey jonasz,

Take a look at the document that I'm including.  Hopefully this will give you a good direction to head in.  Regarding your other question about another way to tell how many items are still in the buffer, there isn't a function that allows you see what's left inside the buffer.